(1) Field of the Invention
The present invention relates to a method for displaying a video image on a digital display device. The invention applies most particularly to apparatus for projection and for back-projection, of television sets or monitors.
(2) Description of Related Art
Among display devices, digital display devices are devices comprising one or more cells which can take a finite number of illumination values. Currently, this finite number of values is equal to two and corresponds to an on state and an off state of the cell. To obtain a larger number of grey levels, it is known to temporally modulate the state of the cells over the video frame so that the human eye, by integrating the pulses of light resulting from these changes of state, can detect intermediate grey levels.
Among the known digital display devices, there are those comprising a digital micromirror matrix or DMD matrix (DMD standing for Digital Micromirror Device). A DMD matrix is a component, conventionally used for video-projection, which is formed of a chip on which are mounted several thousand microscopic mirrors or micromirrors which, controlled on the basis of digital data, serve to project an image onto a screen, by pivoting in such a way as to reflect or to block the light originating from an external source. The technology based on the use of such micromirror matrices and consisting in a digital methoding of light is known as “Digital Light Methoding” or DLP.
In DLP technology, one micromirror per image pixel to be displayed is provided. The micromirror exhibits two operating positions, namely an active position and a passive position, on either side of a quiescent position. In the active position, the micromirror is tilted by a few degrees (around 10 degrees) with respect to its quiescent position so that the light originating from the external source is projected onto the screen through a projection lens. In the passive position, the micromirror is tilted by a few degrees in the opposite direction so that the light originating from the external source is directed towards a light absorber. The periods of illumination of a pixel therefore correspond to the periods during which the associated micromirror is in the active position.
Thus, if the light supplied to the micromirror matrix is white light, the pixels corresponding to the micromirrors in the active position are white and those corresponding to the micromirrors in the passive position are black. The intermediate grey levels are obtained by temporal modulation of the light projected onto the screen corresponding to a PWM modulation (PWM standing for Pulse Width Modulation). Specifically, each micromirror is capable of changing position several thousand times a second. The human eye does not detect these changes of position, nor the light pulses which result therefrom, but integrates the pulses between them and therefore perceives the average light level. The grey level detected by the human eye is therefore directly proportional to the time for which the micromirror is in the active position in the course of a video frame.
To obtain 256 grey levels, the video frame is for example divided into eight consecutive sub-periods of different weights. These sub-periods are commonly called subfields. During each subfield, the micromirrors are either in an active position, or in a passive position. The weight of each subfield is proportional to its duration. FIG. 1 shows an exemplary distribution of the subfields within a video frame. The duration of the video frame is 16.6 or 20 ms depending on the country. The video frame given as an example comprises eight subfields of respective weights 1, 2, 4, 8, 16, 32, 64 and 128. The periods of illumination of a pixel correspond to the subfields during which the associated micromirror is in an active position. The human eye temporally integrates the pixel illumination periods and detects a grey level proportional to the overall duration of the illumination periods in the course of the video frame.
A few problems related to the temporal integration of the illumination periods exist. A problem of false contours appears in particular when an object moves between two consecutive images. This problem is manifested by the appearance of darker or lighter bands on grey level transitions that are normally hardly perceptible. For colour apparatus, these bands may be coloured.
This problem of false contours is illustrated by FIG. 2 representing the subfields for two consecutive images, I and I+1, comprising a transition between a grey level 127 and a grey level 128. This transition moves by 4 pixels between image I and image I+1. In this figure, the ordinate axis represents the time axis and the abscissa axis represents the pixels of the various images. The integration done by the eye amounts to integrating temporally along the oblique lines represented in the figure since the eye tends to follow the object in motion. It therefore integrates information originating from different pixels. The result of the integration is evident through the appearance of a grey level equal to zero at the moment of the transition between the grey levels 127 and 128. This crossing through the zero grey level gives rise to the appearance of a dark band at the level of the transition. In the reverse case, if the transition crosses from the level 128 to the level 127, a level 255 corresponding to a light band appears at the moment of the transition.
A known solution to this problem consists in “breaking” the subfields of high weight so as to decrease the integration error. FIG. 3 represents the same transition as FIG. 2 but with seven subfields of weight 32 instead of the three subfields of weights 32, 64 and 128. The integration error is then at the maximum of a grey level value equal to 32.
It is also possible to distribute the grey levels differently but an integration error still remains.
Furthermore, as in all video appliances, the displaying of a colour image requires the displaying of three images (red, green and blue). In projectors with single DMD matrix, these three images are displayed sequentially. Consequently, such projectors comprise a rotating wheel comprising red, green and blue filters through which the white light originating from the source of the projector is filtered before being transmitted to the DMD matrix. The DMD matrix is thus supplied sequentially with red, green and blue light during the video frame. The rotating wheel generally comprises six filters (2 red, 2 green, 2 blue) and rotates at a frequency of 150 or 180 revs/second, i.e. 3 revolutions per video frame. The digital data of the R, G and B components of the video image are supplied to the DMD matrix in a manner which is synchronized with the red, green and blue light so that the R, G and B components of the image are displayed with the appropriate light. The video frame can therefore be chopped into 18 time segments, 6 for each colour, as illustrated in FIG. 4. In the case of a video frame of 20 ms, the duration of each segment is around 1.1 ms. The subfields shown in FIG. 1 are distributed, for each colour, over the 6 time segments of each colour. The subfield of high weight is for example chopped into six elementary periods each tied to a different time segment whereas the subfield of low weight is present only in one of the 6 segments.
This type of projector exhibits a colour separation defect, also known by the name “colour break-up”, on account of the sequential displaying of the colours. This defect is visible whenever the eye follows a moving object travelling rapidly in the image. The light areas of the image exhibiting a strong contrast then seem to break up momentarily into red, green and blue bands along the direction of motion. Given that the R, G, B components of the image are displayed sequentially and that the eye shifts while following the motion, the colours are therefore reproduced on different locations of the retina of the eye. This therefore prevents the brain from integrating them together into a colour image. Likewise, sudden movements of the eye over a still image may also interrupt the integration of the pulses of light in the brain and disturb the perception of the actual grey level.